fix(js): wrong code for uninitialized pointer locals (#1011)

## Summary

Fix wrong code being generated for uninitialized locals of `pointer`
type, which resulted in run-time crashes or other unexpected
behaviour.

## Details

* the set used in deciding whether to use a boxed pointer missed
  `tyPointer`. Since testing the set after the inclusion of `tyPointer`
  would be redundant with the `mapType` check, testing the set is
  simply removed
* the `not isIndirect` check is replaced with testing whether a boxed
  pointer is used. While a loc using an indirection implies a boxed
  representation, the extra indirection made the logic harder to
  understand

compiler/backend/jsgen.nim

@@ -1673,13 +1673,12 @@ proc genVarInit(p: PProc, typ: PType, varName: string, storage: StorageFlags,
     s: Rope
 
   if n.kind == cnkEmpty:
-    let isIndirect = stfIndirect in storage
-    if not isIndirect and
-      typ.kind in {tyVar, tyPtr, tyLent, tyRef} and mapType(typ) == etyBaseIndex:
+    if mapType(typ) == etyBaseIndex and (stfBoxed notin storage):
       lineF(p, "var $1 = null;$n", [varName])
       lineF(p, "var $1_Idx = 0;$n", [varName])
     else:
-      lineF(p, "var $1 = $2;$n", [varName, createVar(p, typ, isIndirect)])
+      let val = createVar(p, typ, stfIndirect in storage)
+      lineF(p, "var $1 = $2;$n", [varName, val])
   else:
     gen(p, n, a)
     case mapType(typ)
@@ -2293,18 +2292,15 @@ proc finishProc*(p: PProc): string =
     let
       loc {.cursor.} = p.locals[resultId]
       mname = loc.name
-    let returnAddress = not isIndirect(loc) and
-      loc.typ.kind in {tyVar, tyPtr, tyLent, tyRef} and
-        mapType(loc.typ) == etyBaseIndex
-    if returnAddress:
+    if mapType(loc.typ) == etyBaseIndex and (stfBoxed notin loc.storage):
       resultAsgn = p.indentLine(("var $# = null;$n") % [mname])
       resultAsgn.add p.indentLine("var $#_Idx = 0;$n" % [mname])
     else:
       let resVar = createVar(p, loc.typ, isIndirect(loc))
       resultAsgn = p.indentLine(("var $# = $#;$n") % [mname, resVar])
     var a: TCompRes
     accessLoc(mname, loc, a)
-    if returnAddress:
+    if a.typ == etyBaseIndex:
       returnStmt = "return [$#, $#];$n" % [a.address, a.res]
     else:
       returnStmt = "return $#;$n" % [a.res]

tests/js/tdefault_init_pointer_var.nim

@@ -0,0 +1,13 @@
+discard """
+  description: '''
+    Regression test for wrong code being generated for locals of `pointer`
+    type
+  '''
+"""
+
+proc test(): pointer =
+  # the result variable was affected too
+  var x: pointer
+  doAssert x == nil # this assertion failed
+
+doAssert test() == nil # this one too